This invention relates to an electro-mechanical converter utilizing a solenoid coil, and more particularly an electro-mechanical converter wherein a plunger contained in a cylinder is moved in the axial direction in accordance with the magnitude of a control signal supplied to the solenoid coil wound about the cylinder and especially suitable for use as a flow quantity control device of a power steering unit of a motor car, for example.
Since a power steering unit permits easy and rapid steering of a heavy car with an extremely small power as well as a stable running due to its servoeffect and cushioning effect even when the steering wheels of the car receive lateral shock from the load surface, power steering units having various constructions have been developed. According to one example of the prior art constructions, a servovalve utilizing a solenoid coil is connected in a passage of an operating fluid utilized for the power steering unit so as to control the flow of the operating fluid in accordance with the running speed of the motor car. This construction increases the steering power at the high speed running over that of the low speed running thus giving to the driver a steering feeling commensurate with the running speed.
One example of a prior art servovalve utilizing a solenoid coil is disclosed in Japanese laid open patent specification No. 108732/74 wherein a plunger with one end connected to a needle of a needle control valve is contained in a cylinder and an electric coil is wound about the cylinder. A current corresponding to the running speed of a motor car is supplied to the coil to move the plunger in the axial direction for controlling the flow of the operating fluid supplied to the power steering unit through the needle control valve.
In such a servovalve, the plunger is generally made of magnetic material and its external diameter is designed close to the inner diameter of the cylinder so as to closely couple the plunger with magnetic flux generated by the solenoid coil. Further, a portion of the cylinder is also made of magnetic material and designed to bring its end as close as possible to the end of the plunger. Consequently, the plunger is caused to slide along the inner wall of the cylinder by the magnetic flux created by the solenoid coil in response to the control signal. For this reason, a frictional resistance is created between the plunger and the cylinder whereby when the plunger is reciprocated by the solenoid coil and by a return spring, a hysteresis occurs as shown in FIG. 1 of the accompanying drawing in which the ordinate represents the displacement of the plunger and the abscissa represents a control current supplied to the solenoid coil. With the prior art electro-mechanical converter, as shown by curve a, as the control signal is increased gradually, the plunger moves greatly from point P.sub.1 to point P.sub.2 at a current value i.sub. 1 and then the plunger moves gradually to point P.sub.3 as the current is increased to i.sub.2. Thereafter, the plunger does not move beyond point P.sub.3 even when the current is increased. Conversely, when the current is decreased, the plunger does not move between current values i.sub.1 and i.sub.2 but moves greatly from point P.sub.3 to Point P.sub.4 at a current i.sub.3 smaller than i.sub.1 as shown by curve b. Thereafter, the plunger moves gradually to the initial point P.sub.1 as the current is decreased to i.sub.4. Accordingly, when an electro-mechanical converter having such displacement-control signal characteristic is used, it is difficult to provide linear control due to the frictional resistance between the cylinder and the plunger. Moreover, the amount of displacement of the plunger is small. These defects limit the field of application of the electro-mechanical converter, so that its use has been limited to only on-off control.
When an electro-mechanical converter having such a hysteresis characteristic is used in a power steering unit for the control of the flow, the handle becomes rapidly light or heavy at points where the flow quantity changes. Moreover, as such points vary depending upon the direction of rotating the steering handle the steering sense of the driver would be impaired.
FIG. 2 shows a flow quantity-running speed characteristic of a control valve utilized in the power steering unit in which the abscissa represents running speed of a motor car, while the ordinate represents the flow quantity of the operating fluid supplied to the power steering unit through a needle valve. As shown by curve a, with a conventional servovalve, as the running speed of the motor car increases the flow greatly decreases from F.sub.1 to F.sub.2 at speed V.sub.1, then gradually decreases to F.sub.3 until speed V.sub.2 is reached. Thereafter, the flow is maintained at F.sub.3 even when the speed increases. Conversely, when the speed is decreased from high to low, as shown by curve b the flow does not vary at speeds V.sub.1 and V.sub.2, but greatly increases at V.sub.3 and then gradually increases to the initial value F.sub.1 until speed V.sub.4 is reached. Accordingly, with a power steering unit having such characteristic the flow varies greatly at speeds V.sub.1 and V.sub.3 with the result that the steering handle of the car becomes abruptly light or heavy at points where the flow changes greatly. Moreover, as such flow quantity varying points differ depending upon the direction of rotating the steering wheel, the steering sense of the driver is impaired.
Such rapid change in the steering load was also experienced in a power steering unit in which the fluid pressure is controlled by the electro-mechanical converter described above.